Thermal ground plane

ABSTRACT

Methods, apparatuses, and systems are disclosed for flexible thermal ground planes. A flexible thermal ground plane may include a support member. The flexible thermal ground plane may include an evaporator region or multiple evaporator regions configured to couple with the support member. The flexible thermal ground plane may include a condenser region or multiple condenser regions configured to couple with the support member. The evaporator and condenser region may include a microwicking structure. The evaporator and condenser region may include a nanowicking structure coupled with the micro-wicking structure, where the nanowicking structure includes nanorods. The evaporator and condenser region may include a nanomesh coupled with the nanorods and/or the microwicking structure. Some embodiments may include a micromesh coupled with the nanorods and/or the microwicking structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/681,624, filed on Apr. 8, 2015, titled “Flexible Thermal Ground Planeand Manufacturing the Same,” which is a continuation of U.S. patentapplication Ser. No. 12/719,775, filed on Mar. 8, 2010, titled “FlexibleThermal Ground Plane and Manufacturing the Same,” which is anon-provisional claiming priority benefit of U.S. provisional patentapplication Ser. No. 61/158,086, filed on Mar. 6, 2009, titled “FlexibleThermal Ground Plane and Manufacturing the Same and Means to Fabricate aFlexible Thermal Ground Plane,” the entire disclosure of which is hereinincorporated by reference for all purposes.

This invention was made with government support under Grant No.N66001-08-C-2006 awarded by DOD/DARPA and Grant No. HR0011-06-1-0048awarded by the DOD/DARPA. The government has certain rights in theinvention.

BACKGROUND

This application relates generally to thermal ground planes. Morespecifically, this application relates to methods, apparatuses, andsystems for flexible thermal ground planes.

The complexity and size of integrated circuits may been limited by theheat at generated. Heat pipes have been used to transfer heatefficiently from one location to another. They have also been used tocool integrated circuits. The existing heat pipes for these purposes mayconsist of a rigid structure composed of copper, silicon, etc. Somemodern electrical devices and systems demand a flexible circuit boardalong with a high capacity for heat dissipation.

There is thus a need for methods, systems, and devices that may also beflexible while transferring heat efficiently from one location toanother or spread high flux heat from a small area to low heat flux overa larger area.

SUMMARY

Embodiments of the invention include a method for certain embodimentsthus provide methods, systems, and devices that may include a flexiblethermal ground plane. Embodiments of flexible thermal ground planes mayprovide extremely high thermal performance with highevaporation/condensation heat transfer and effective liquid supply.Flexible configurations may be enabled by using polymer casing laminatedand covered by moisture barrier coatings, enabled by atomic layerdeposition, chemical vapor deposition, physical vapor deposition, orthin metal laminate, merely by way of example. Embodiments of flexiblethermal ground planes may also involve low cost construction resultingfrom large size manufacturing, e.g. 3 ft wide and 1000 ft long, merelyby way of example. For example, flexible thermal ground planeconstruction may take advantage of flexible circuit board manufacturingtechnology. Large size flexible thermal ground planes may thus beconstructed for some embodiments, e.g. 20 cm by 40 cm by 1 mm, merely byway of example.

Methods, apparatuses, and systems are disclosed for flexible thermalground planes. A flexible thermal ground plane may include a supportmember. The flexible thermal ground plane may include an evaporatorregion configured to couple with the support member. The evaporatorregion may include a micro-wicking structure. The evaporator region mayinclude a nanowicking structure coupled with the micro-wickingstructure, where the nanowicking structure includes nanorods. Theevaporator region may include a micro-mesh or nano-mesh coupled with themicro-wicking or nanowicking structure.

Some embodiments may include a flexible thermal ground plane. Theflexible thermal ground plane may include a flexible support member. Theflexible support member may be configured to enclose a working fluid.One or more evaporators regions may couple with the flexible supportmember. One or more condenser regions may couple with the flexiblesupport. The flexible thermal ground plane may include a working fluidthat is enclosed by the flexible support member.

In some embodiments of the flexible thermal ground plane, the supportmember may include a polymer layer. The polymer layer may include aliquid crystal polymer. In some embodiments of the flexible thermalground plane, the support member may include a thin metal layer. In someembodiments, flexible thermal ground plane may include a moisturebarrier layer coupled with the polymer layer. The moisture barrier layermay include an atomic layer deposition coating, a chemical vapordeposition coating, a physical vapor deposition coating or thin metallaminate coating.

In some embodiments, the evaporator and/or the condenser region mayinclude a microwicking structure. In some embodiments, the evaporatorand/or condenser region may include a nanowicking structure. In someembodiments, the evaporator and the condenser region may include ahybrid micro/nano wicking structure. The nanowicking structure mayinclude nanorods.

In some embodiments, the flexible thermal ground plane may include amesh layer coupled with the evaporator region and/or the condenserregion. In some embodiments of the flexible thermal ground plane, a meshlayer may be included and the mesh layer separates a vapor chamber froma liquid channel. In some embodiments, the mesh layer may include ananomesh layer.

Some embodiments may include a thermal ground plane system. The thermalground plane system may include a support member. The thermal groundplane system may include an evaporator region configured to couple withthe support member. The evaporator region may include a first wickingstructure. The thermal ground plane system may include a condenserregion configured to couple with the support member. The condenserregion may include a second wicking structure.

In some embodiments, the thermal ground plane system may also include amesh structure coupled with the first wicking structure and the secondwicking structure.

In some embodiments of a thermal ground plane system, at least one ofthe first wicking structure or the second wicking structures includes amicrowicking structure. In some embodiments of a thermal ground planesystem, at least one of the first wicking structure or the secondwicking structures includes a nanowicking structure. In someembodiments, the thermal ground plane system may include a third wickingstructure, where the third structure couples with the first wickingstructure and the mesh structure. The third wicking structure mayinclude a microwicking structure or a nanowicking structure. The thirdwicking structure may include nanorods and/or a nanomesh. In someembodiments, the thermal ground plane system may include a moisturebarrier coating coupled with the support member. In some embodiments,the support member may include a flexible polymer member.

In some embodiments, the thermal ground plane system may include highthermal conductivity thermal vias coupled with the support member. Insome embodiments, the thermal ground plane system may include a highthermal conductivity member coupled with the support member, wherein atleast one of the first wicking structure or second wicking structure ismade on the high thermal conductivity member.

Some embodiments may include method of fabricating a thermal groundplane. The method may include providing a support member. The method mayinclude coupling a plurality of microwicking structures with a surfaceof the support member.

In some embodiments, the method of fabricating a thermal ground planemay include a support member that includes a flexible member. Theflexible member may include a polymer layer. The polymer layer mayinclude a liquid crystal polymer. The flexible member may be a thinmetal layer in some embodiments.

In some embodiments, the method of fabricating a thermal ground planemay include coupling a mesh structure to the plurality of microwickingstructures. In some embodiments, the method of fabricating a thermalground plane may include coupling a nanowicking structure between themicrowicking structure and the mesh structure. In some embodiments, themethod of fabricating a thermal ground plane may include coupling a meshstructure with the plurality of nanowicking structures. In someembodiments, the method of fabricating a thermal ground plane mayfurther include coupling a microwicking structure between at least oneof the microwicking structures and the mesh structure.

In some embodiments, the method of fabricating a thermal ground planemay further include coupling a plurality of nanowicking structures withthe surface of the support member. In some embodiments, the method offabricating a thermal ground plane may include coupling a secondmicrowicking structure with at least one of the plurality ofmicrowicking structures. In some embodiments, the method of fabricatinga thermal ground plane may include coupling a nanowicking structure withat least one of the plurality of microwicking structures.

In some embodiments, the method of fabricating a thermal ground planemay include applying a coating to the mesh structure, where the coatingcreates at least one hydrophobic region or one hydrophilic region. Insome embodiments, the method of fabricating a thermal ground plane mayinclude applying a coating to the mesh structure, wherein the coatingcreates corrosion protection for the thermal ground plane. In someembodiments, the method of fabricating a thermal ground plane mayinclude creating a moisture barrier coupled with the support member.Creating a moisture barrier may include forming a moisture barriercoating using at least one of atomic layer deposition, chemical vapordeposition, physical vapor deposition, or thin metal lamination.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described in conjunction with the appendedfigures:

FIG. 1 provides a diagram of a thermal ground plane, in accordance withvarious embodiments.

FIG. 2 provides a diagram of a thermal ground plane, in accordance withvarious embodiments.

FIG. 3 provides a diagram showing various aspects of a thermal groundplane, in accordance with various embodiments.

FIG. 4 shows diagrams for operating a thermal ground plane and a methodof fabrication, in accordance with various embodiments.

FIG. 5 shows aspects of a thermal ground plane involving an atomic layerdeposition, in accordance with various embodiments.

FIG. 6 provides data regarding aging effect on permeability for somethermal ground planes involve atomic layer deposition, in accordancewith various embodiments.

FIGS. 7-a and 7-b show different micro/nano-scaled evaporator structuresfabricated on liquid crystal polymers, in accordance with variousembodiments.

FIG. 8 provides some experimental data regarding critical heat fluxesregarding evaporation or condensation, in accordance with variousembodiments.

FIG. 9 shows a one-dimensional heat transfer model of a thermal groundplane, in accordance with various embodiments.

FIG. 10 provides some results regarding saturation temperature andevaporation heat trans coefficients, in accordance with variousembodiments.

FIG. 11 provides heat flux data regarding evaporators, in accordancewith various embodiments.

FIGS. 12-a and 12-b provide data regarding working temperatures andcapillary pressures for some embodiments.

FIGS. 13-a and 13-b provide information regarding thermal conductivityas function of length for some embodiments.

FIG. 14 shows some structural and property features of a thermal groundplane, in accordance with various embodiments.

FIG. 15 shows aspects of some thermal ground planes, in accordance withvarious embodiments.

FIG. 16 shows defects that may be identified for some thermal groundplanes that include atomic layer deposition, in accordance with variousembodiments.

FIGS. 17-a, 17-b, and 17-c show micro/nano-scaled evaporator structures,in accordance with various embodiments.

FIG. 18 shows aspects of some fabrication of thermal ground planeinvolving copper through hole vias, in accordance with variousembodiments.

FIGS. 19-a and 19-b show methods of fabrication of thermal groundplanes, in accordance with various embodiments.

FIGS. 20-a and 20-b show micro/nano wicking structures, in accordancewith various embodiments.

FIG. 21 shows a thermal ground plane with a micro-meshed copper sheet,in accordance with various embodiments.

FIG. 22 shows aspects of fabricating a thermal ground plane, inaccordance to various embodiments.

FIG. 23 shows aspects of fabricating a thermal ground plane, inaccordance to various embodiments.

FIG. 24 shows aspects of fabricating a thermal ground plane, inaccordance to various embodiments.

FIG. 25 shows a cross section of micro-vias of a thermal ground plane,in accordance to various embodiments.

FIG. 26 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 27 shows a cross section of copper filled micro-vias of a thermalground plane, in accordance with various embodiments.

FIG. 28 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 29 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 30 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 31 provides an image of a micro-cube evaporator structure, inaccordance with various embodiments.

FIG. 32 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 33 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 34 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 35 shows a micro-mesh wicking structure, in accordance with variousembodiments.

FIG. 36 shows a cross section of mesh bonded to micro-channels andfilled thermal vias, in accordance with various embodiments.

FIG. 37 shows additional aspects of fabricating a thermal ground plane,in accordance to various embodiments.

FIG. 38 shows a thermal ground plane vehicle, in accordance with variousembodiments.

FIG. 39 shows a thermal ground plane, in accordance with variousembodiments.

FIG. 40 provides some results regarding effective thermal conductivityfor a thermal plane device, according to various embodiments.

In the appended figures, similar components and/or features may have thesame numerical reference label. Further, various components of the sametype may be distinguished by following the reference label by a letterthat distinguishes among the similar components and/or features. If onlythe first numerical reference label is used in the specification, thedescription is applicable to any one of the similar components and/orfeatures having the same first numerical reference label irrespective ofthe letter suffix.

DETAILED DESCRIPTION

The ensuing description provides exemplary embodiments only, and is notintended to limit the scope, applicability or configuration of thedisclosure. Rather, the ensuing description of the exemplary embodimentswill provide those skilled in the art with an enabling description forimplementing one or more exemplary embodiments. It being understood thatvarious changes may be made in the function and arrangement of elementswithout departing from the spirit and scope of the invention as setforth in the appended claims.

Specific details are given in the following description to provide athorough understanding of the embodiments. However, it will beunderstood by one of ordinary skill in the art that the embodiments maybe practiced without these specific details. For example, circuits,systems, networks, processes, and other elements in the invention may beshown as components in block diagram form in order not to obscure theembodiments in unnecessary detail. In other instances, well-knowncircuits, processes, algorithms, structures, and techniques may be shownwithout unnecessary detail in order to avoid obscuring the embodiments.

Also, it is noted that individual embodiments may be described as aprocess which is depicted as a flowchart, a flow diagram, a data flowdiagram, a structure diagram, or a block diagram. Although a flowchartmay describe the operations as a sequential process, many of theoperations can be performed in parallel or concurrently. In addition,the order of the operations may be re-arranged. A process may beterminated when its operations are completed, but could have additionalsteps not discussed or included in a figure. Furthermore, not alloperations in any particularly described process may occur in allembodiments. A process may correspond to a method, a function, aprocedure, a subroutine, a subprogram, etc. When a process correspondsto a function, its termination corresponds to a return of the functionto the calling function or the main function.

Certain embodiments thus provide methods, systems, and devices that mayinclude a flexible thermal ground plane. Embodiments of flexible thermalground planes may provide extremely high thermal performance with highevaporation/condensation heat transfer and effective liquid supply.Flexible configurations may be enabled by using polymer casing laminatedand covered by moisture barrier coatings, enabled by atomic layerdeposition, chemical vapor deposition, physical vapor deposition, orthin metal laminate, merely by way of example. Embodiments of flexiblethermal ground planes may also involve low cost construction resultingfrom large size manufacturing, e.g. 3 ft wide and 1000 ft long, merelyby way of example. For example, flexible thermal ground planeconstruction may take advantage of flexible circuit board manufacturingtechnology. Large size flexible thermal ground planes may thus beconstructed for some embodiments, e.g. 20 cm by 40 cm by 1 mm, merely byway of example.

Methods, apparatuses, and systems are disclosed for flexible thermalground planes. A flexible thermal ground plane may include a supportmember. The flexible thermal ground plane may include an evaporatorregion configured to couple with the support member. The evaporatorregion may include a micro-wicking structure. The evaporator region mayinclude a nanowicking structure coupled with the micro-wickingstructure, where the nanowicking structure includes nanorods. Theevaporator region may include a micro-mesh or nanomesh coupled with themicro-wicking or nanowicking structure.

Some embodiments may include a flexible thermal ground plane. Theflexible thermal ground plane may include a flexible support member. Theflexible support member may be configured to enclose a working fluid.One or more evaporators regions may couple with the flexible supportmember. One or more condenser regions may couple with the flexiblesupport. The flexible thermal ground plane may include a working fluidthat is enclosed by the flexible support member.

In some embodiments of the flexible thermal ground plane, the supportmember may include a polymer layer. The polymer layer may include aliquid crystal polymer. In some embodiments of the flexible thermalground plane, the support member may include a thin metal layer. In someembodiments, flexible thermal ground plane may include a moisturebarrier layer coupled with the polymer layer. The moisture barrier layermay include an atomic layer deposition coating, a chemical vapordeposition coating, a physical vapor deposition coating or thin metallaminate coating.

In some embodiments, the evaporator and/or the condenser region mayinclude a microwicking structure. In some embodiments, the evaporatorand/or condenser region may include a nanowicking structure. In someembodiments, the evaporator and the condenser region may include ahybrid micro/nano wicking structure. The nanowicking structure mayinclude nanorods.

In some embodiments, the flexible thermal ground plane may include amesh layer coupled with the evaporator region and/or the condenserregion. In some embodiments of the flexible thermal ground plane, a meshlayer may be included and the mesh layer separates a vapor chamber froma liquid channel. In some embodiments, the mesh layer may include ananomesh layer.

Some embodiments may include a thermal ground plane system. The thermalground plane system may include a support member. The thermal groundplane system may include an evaporator region configured to couple withthe support member. The evaporator region may include a first wickingstructure. The thermal ground plane system may include a condenserregion configured to couple with the support member. The condenserregion may include a second wicking structure.

In some embodiments, the thermal ground plane system may also include amesh structure coupled with the first wicking structure and the secondwicking structure.

In some embodiments of a thermal ground plane system, at least one ofthe first wicking structure or the second wicking structures includes amicrowicking structure. In some embodiments of a thermal ground planesystem, at least one of the first wicking structure or the secondwicking structures includes a nanowicking structure. In someembodiments, the thermal ground plane system may include a third wickingstructure, where the third structure couples with the first wickingstructure and the mesh structure. The third wicking structure mayinclude a microwicking structure or a nanowicking structure. The thirdwicking structure may include nanorods and/or a nanomesh. In someembodiments, the thermal ground plane system may include a moisturebarrier coating coupled with the support member. In some embodiments,the support member may include a flexible polymer member.

In some embodiments, the thermal ground plane system may include highthermal conductivity thermal vias coupled with the support member. Insome embodiments, the thermal ground plane system may include a highthermal conductivity member coupled with the support member, wherein atleast one of the first wicking structure or second wicking structure ismade on the high thermal conductivity member.

Some embodiments may include method of fabricating a thermal groundplane. The method may include providing a support member. The method mayinclude coupling a plurality of microwicking structures with a surfaceof the support member.

In some embodiments, the method of fabricating a thermal ground planemay include a support member that includes a flexible member. Theflexible member may include a polymer layer. The polymer layer mayinclude a liquid crystal polymer. The flexible member may be a thinmetal layer in some embodiments.

In some embodiments, the method of fabricating a thermal ground planemay include coupling a mesh structure to the plurality of microwickingstructures. In some embodiments, the method of fabricating a thermalground plane may include coupling a nanowicking structure between themicrowicking structure and the mesh structure. In some embodiments, themethod of fabricating a thermal ground plane may include coupling a meshstructure with the plurality of nanowicking structures. In someembodiments, the method of fabricating a thermal ground plane mayfurther include coupling a microwicking structure between at least oneof the microwicking structures and the mesh structure.

In some embodiments, the method of fabricating a thermal ground planemay further include coupling a plurality of nanowicking structures withthe surface of the support member. In some embodiments, the method offabricating a thermal ground plane may include coupling a secondmicrowicking structure with at least one of the plurality ofmicrowicking structures. In some embodiments, the method of fabricatinga thermal ground plane may include coupling a nanowicking structure withat least one of the plurality of microwicking structures.

In some embodiments, the method of fabricating a thermal ground planemay include applying a coating to the mesh structure, where the coatingcreates at least one hydrophobic region or one hydrophilic region. Insome embodiments, the method of fabricating a thermal ground plane mayinclude applying a coating to the mesh structure, wherein the coatingcreates corrosion protection for the thermal ground plane. In someembodiments, the method of fabricating a thermal ground plane mayinclude creating a moisture barrier coupled with the support member.Creating a moisture barrier may include forming a moisture barriercoating using at least one of atomic layer deposition, chemical vapordeposition, physical vapor deposition, or thin metal lamination.

FIG. 1 provides an example of a system or device 100 involving aflexible thermal ground plane configuration in accordance with variousembodiments. System 100 includes one or more substrate or supportmembers 110-i, 100-j. In some embodiments, substrate member may beflexible. A flexible substrate may be made out of numerous differentmaterials including, but not limited to thin metal layer, liquid crystalpolymers, Kapton, or other polymers materials.

System 100 also shows one or more evaporator regions 120-i, 120-j and/orone or more condenser regions 130-i, 130-j coupled with the supportmember. Evaporator regions 120 and condenser regions 130 may be also bereferred to as evaporator components or condenser componentsrespectively. Evaporator and condenser components 120, 130 respectivelymay include metal or thermally conductive materials. They may comprisemicrostructures and/or nanostructures. Merely by way of example,evaporator region shows a micro structure coupled with a nanostructure.The nanostructure may include nanorods or nanomeshes. Coupled with theevaporator components and/or condenser components may also be meshstructure 140-i, 140-j. Mesh structure 140 may be a micro-mesh structurein some embodiments. Mesh structure 140 may be a nanomesh structure insome embodiments.

System 100 shows that mesh structure 140 may separate a vapor core 150from a liquid channel 160. Mesh structure 140 may be a micro-meshstructure in some embodiments. Mesh structure 140 may be a nanomeshstructure in some embodiments. A liquid, such as distilled water merelyby way of example, may flow through the liquid channel 160, which may bepart of a micro-structure layer of the system for some embodiments. Atevaporator region 120, the liquid may be heated up as it pass through oraround micro and/or nanostructures of evaporator region 120. The liquidmay then pass through mesh structure 140 and become a vapor. Within thevapor core 150, the vapor may then condense back to a liquid atcondenser region 130, passing back through mesh structure 140 andthrough and/or around micro and/or nanostructures of the condenserregion 130.

System 100 also shows moisture barrier 170 coupled with substrate member110. Moisture barrier 170 may be a moisture barrier coating that may beformed in a variety of different ways including, but not limited to,atomic layer deposition, chemical vapor deposition, physical vapordeposition, or thin metal lamination. Moisture barrier 170 may be usedin some embodiments to retain liquid and/or vapor within system 100. Forexample, substrate member 110 may be porous enough to allow for liquidto pass through it. With moisture barrier 170, liquid can be kept withinthe system. In some embodiments, thermal vias (not shown) made ofmaterials with a thermal conductivity much higher than that of thesubstrate material may be included in substrate member 110. Thermal viasmay aid in transferring heat from a source (not shown) to a flexiblethermal ground plane system or device such as system 100. Someembodiments may include one or more connection layers 190.

FIG. 2 provides a system 200 involving a flexible thermal ground plane,which may include a flexible thermal ground plane system such as system100. System 200 shows numerous evaporator 120 and condenser regions 130coupled with substrate 110. Evaporator regions and condenser regions maybe placed in numerous different locations on a support member. Theplacement of the evaporator and/or condenser regions may depend on thespecific needs and/or configurations of other components that system 200may couple with, for example.

Embodiments shown in FIG. 3 include methods for micro/nanofabrication todevelop a flexible thermal ground plane (FTGP), which may serve as astand-alone component or may be embedded in a flexible or rigidsubstrate for effective thermal management for some embodiments. FIG. 3shows FTGP with micro/nano-scaled wicking structure made of copper orother solid materials for distilled water or other liquid. The flexiblethermal ground plane may made hermetic through the use of liquid crystalpolymer (LCP) with low moisture permeability, encapsulated by anano-scaled alumina coating through atomic layer deposition (ALD). Someembodiments may thus include a flexible substrate or support member. Theflexible substrate or support member may include a polymer material suchas LCP or polyimide, merely by way of example. Embodiments may alsoinclude a moisture barrier, which may be a coating. Moisture barriersmay include, but are not limited to ALD, chemical vapor deposition,physical vapor deposition, or thin metal laminate. The micro/nano wickstructures may incorporate an ALD hydrophilic coating for the evaporatorand an ALD hydrophobic coating for the condenser to assure effectiveoperation under high heat flux conditions. The micro/nano wickstructures may incorporate an ALD coating for liquid-corrosionprotection on evaporator or condenser.

In some embodiments, micro/nano wicks may be realized as copper nanorodsand nanomeshes electroplated or etched on the LCP's copper microcubesand channels through the use of nanofabrication templates, such as blockcopolymer templates, porous anodized alumina templates or colloidalparticle polymer template, merely by way of example. The copper-basedwicks may have very high thermal conductivity. With extremely efficientevaporators and condensers for water, the FTGP's thermal conductivitymay be at least 100× higher than that of current common copper-alloysubstrates

In some embodiments, an FTGP may be fabricated using LCP flexiblecircuit technologies; it may be extremely thin (<1 mm) and low weight,while covering a large area. The operation of the FTGP may be simple androbust with a long lifetime.

FTGPs may be fabricated using LCP+ALD hermetic sealing, coppermicro/nano wick structures with nanomesh and nanorods fabricated onmicrocubes on LCP or polyimide flexible circuits. Merely by way ofexample, an integrated 3 cm×3 cm×3 mm FTGP sample may be constructed andmay demonstrate >500 W/mK performance. Refinements to the design andfabrication of FTGP's components may be carried out to improveperformance (>30,000 W/rnK) and reliability. Merely by way of example,20 cm×40 cm×0.9 mm FTGP samples may be produced. Other dimensioned FTGPsmay also be produced to fit the requirements of different applications

Merely by way of example, the following Table 1 provides some of theinnovations that may be seen with different embodiments of FTGP:

TABLE 1 Metrics Goals SOA Innovative Claims Hermeticity 0.1% fluid 0%LCP has low water vapor permeability, which may be reduced by theloss/year addition of ALD alumina coating to reach 1.7 × 10⁻⁵*g/m² day(50,000X @ 100° C. improvement over typical polymer). Wicking* 25 gforce at 3 g Nanopores (400 nm) and ALD superhydrophilic coating mayincrease 80° C.* the capillary pumping pressure by 800X and may overcomeflow resistance under a 25 g condition. Wick Thermal 100 W/m.K  23 Useof Cu with K = 400 W/mK for micro/nano wicking structure mayConductivity assure wick thermal conductivity higher than 100 W/mK. TOPThermal 30,000 200 Nanopores (400 nm) and ALD superhydrophilic coatingmay increase Conductivity* W/mK for evaporation heat transfer by 100X(2500 kW/m²K), while nanopores 10 cm × (400 nm) and ALD superhydrophobiccoating may enhance 20 cm condensation by 30X for the target 600 kW/m²K.Critical heat flux may TGP* reach >1,000 W/cm². TGP 0.9 mm  2 LCPflexible circuit technology may be ideal to fabricate an extremelyThickness thin FTGP to be laminated with MCM-L. Area* 20 cm 15 × 30Micro/nano wick structures, compatible with the flexible circuit and X40cm* adhesive spacer technology developed for a prior DARPA MEMS effort,may assure successful operation over a large area. Weight 50 gm 270 Theflexible circuit-based thin FTGP may be low weight. Duration 1000 hoursInfinite The 500 μm vapor core may be 20X larger than the 151 μm limitfor vapor continuum operation; duration will be > 1000 hrs.

Some embodiments of a FTGP may be superior to state-of-the-art (“SOA”)flat heat pipes with the performance listed above. In addition, the FTGPmay have excellent manufacturability and flexibility. In someembodiments, in one spool, merely by way of example, one may printcircuits 3 feet wide and 1000 ft long with the lowest manufacturing costpossible. Embodiments may utilize flexible circuit manufacturingtechniques and equipment. Flexible modules including electronics,optoelectronics, RF and sensors/actuators may be designed, customized,and/or tailored to accommodate different size and geometricrequirements. Embodiments may include flexible modules with mixeddevices. These modules' thermal performance may be increased by 1000×with a FTGP, merely by way of example.

In some embodiments, FTGPs may be enabled by the following technologies,though other methods, techniques, and technologies may also be usedwithin the spirit of the invention: maskless fabrication of themicro/nano wicking structure, atomic layer deposition (“ALD”) forhydrophilic, hydrophobic, hermetic and corrosion-protective coatings.These technologies may be described as follows.

In some embodiments, maskless fabrication of the hybridmicro/nano-scaled wicking structure for distilled water or other liquid,merely by way of example, may enable the FTGP to reach: a) ultrahighheat transfer coefficients in both evaporation and condensation, b) lowflow resistance, and/or c) high capillary pressure to sustain operationin a high-g acceleration environment. FIG. 4, for example, showsembodiments of an FTGP operation principle and a fabrication concept ofthe nanorods/nanomeshes. Maskless, low-cost and scaleablenano-fabrication using block copolymer as plating/etching templates maybe compatible with current flexible circuit manufacturing, theresolution of which is typically limited to 25 μm lines/spacings.

FIG. 4 shows embodiments of a FTGP, where on the left, the liquid may besupplied by the capillary force difference between the evaporator andthe condenser in some embodiments. Distilled water or other liquid mayflow through the micro-scaled features to the nano-scaled surface foreffective evaporation. The vapor may condense on the hydrophobicsurfaces and the droplets may be absorbed by the hydrophilic wick.Condensate may be pumped back to the evaporator by the capillarypressure, completing the liquid-vapor-liquid cycle and starting the nextone.

Referring again to FIG. 4, embodiments of a micro/nano wicking structuremay be shown on the right. Nanomeshes may form a membrane that separatesthe vapor core from the liquid. Nanomeshes and nanorods may be etched orplated using nanofabrication templates, such as block copolymertemplates, porous alumina templates or colloidal particle templates, formaskless nano-fabrication.

In some embodiments, atomic layer deposition (“ALD”) may provide forhydrophilic and hydrophobic coatings on the micro/nano-scaled wickingstructures, due to the extremely low intrinsic thermal resistance of thenanoscale-thick coatings which can be (created/produced). To obtain ahydrophilic coating with small contact angle in water, some embodimentsmay apply ALD SiO₂ and TiO₂ coatings merely by way of example on thewicking structures of the evaporator. For a hydrophobic coating on thecondenser, ALD alkylaminosilanes polymer may be formed with covalentbonding to the hydroxyl groups on an ALD alumina layer, as shown in FIG.5, attached to the nanorods/nanomeshes. The contact angle with water ona planar surface may be about 108° in some embodiments, and may reach160° or larger for ALD superhydrophobic nanostructures. Moreover, ALDalumina coating may provide a water vapor barrier for some embodimentsas shown in FIG. 6. Merely by way of example, a water vapor permeabilitymay be 1.7×10-5 gm/m²·day in some embodiments, which is about 50,000×lower than the 1 gm/m²·day for a typical polymer. The aging effect onthe permeability as shown in FIG. 6 may be overcome by applyingadditional ALD SiO₂ or fluoropolymer on top of ALD alumina. Moreover,ALD coating may provide a corrosion protective coating that preventsliquid corrosion on the wicking structures.

In some embodiments, other thin film deposition techniques may be usedbesides and/or in addition to ALD. Merely by way of example, chemicalvapor deposition (“CVD”) techniques may be used in some embodiments. CVDtechniques may include, but are not limited to, the following:atmospheric CVD, low-pressure CVD, ultrahigh vacuum CVD, aerosolassisted CVD, direct liquid injection CVD, microwave plasma-assistedCVD, plasma-enhanced CVD, remote plasma-enhanced CVD, atomic layer CVD,hot wire CVD, metal organic CVD, hybrid physical-chemical CVD, rapidthermal CVD, and/or vapor phase epitaxy. Merely by way of example,physical vapor deposition (“PVD”) techniques may be in used in someembodiments. PVD techniques may include, but are not limited to, thefollowing: evaporative deposition, electron beam PVD, sputterdeposition, cathodic arc deposition, pulsed laser deposition, ionassisted deposition, and/or ion plating. Other thin film depositiontechniques that may be used in some embodiments also includeelectroplating, electrodeposition, thermal oxidation, sputtering,reactive sputtering, evaporation, casting, molecular beam epitaxy,vapor-phase epitaxy, liquid-phase epitaxy, solid-phase epitaxy,homoepitaxy, heteroepitaxy, and/or heterotopotaxy. In some embodiments,a lamination of a metal layer be utilized.

Embodiments may involve a polymer including liquid crystal polymer(LCP). Liquid crystal polymer may have low moisture permeability, whichmay provide for hermetic sealing for extremely low fluid loss in someembodiments. LCP may be used as an FTGP casing material in someembodiments. Polyimide material commonly used for fabricating flexiblecircuit boards can be good alternative to LCP.

Merely by way of example, FIG. 7-a shows an approach with themicro/nano-scaled evaporator structure fabricated on LCP, which includesa chip-on-chip configuration for thermally matched chip attached to LCP.The evaporator may be thermally connected to a dummy intermediate chipthrough thermal vias and copper pads. The effective coefficients ofthermal expansion (CTE) of the dummy chip may be collectively influencedby the LCP and the chip attachment material. For high heat fluxapplications, embodiments may apply the configuration shown in FIG. 7-b,in which the evaporator structure is fabricated on the real chip and mayprovide additional thermal performance.

FTGPs may be utilized with many different systems. Merely by way ofexample, FTGPs may be utilized with (a) laser modules representing highheat flux applications, (b) transmitter/receiver (T/R) modulesrepresenting high power applications, (c) power control boardsrepresenting folded flexible-circuits applications, and/or (d)concentrated solar power plants. FTGPs may be utilized with many othersystems where thermal management is a concern as one skilled in the artwill recognize

Merely by way of example, with the miniaturization of electronic systemsand increasing heat loads, thermal management may be one of the mostcritical sub-systems of military systems. Embodiments of FTGP mayprovide improved performance for these systems and other over othercurrent and future solutions. FTGP may be applicable with many differenttypes of components. For example, electronic components may include newfamilies of chips 1000 with heat flux levels ranging from 50 W/cm² to150 W/cm², and laser diodes ranging from 150 W/cm² to ˜1000 W/cm². Morespecifically, these electronic components may be for high densitymicro/nano spacecraft electronics, high power electronics, and laserdiode arrays for both DoD and NASA Earth observing or science missions.Heat pipes may have been applied to these components already. Whileimprovements on heat pipes and incorporation of active liquid coolingsystems are being investigated, FTGP may out perform all these currentand future solutions. FTGP may also be applicable for other forms oftechnologies, include battery and other energy storage technologies ingeneral.

Merely by way of example, the following table may provide comparison ofFTGP and state-of-the-art vapor chambers (also called flat heat pipes)reported by some vendors. The wicking structures of state-of-the-artvapor chambers may be made from sintered copper or other metals, merelyby way of example. With the development of micro/nano hybrid wickingstructure, FTGP's thermal conductivity may be at least 10× higher thanthose achieved in rigid vapor chambers today, with an order-of-magnitudeweight reduction as well. The FTGP's heat flux may also reach as high as1,000 W/cm² with a very dramatic thickness reduction, which would be anenabling technology for future military electronic devices and systems.

In addition, merely by way of example, the following table compares FTGPwith some recent research on micro-scale flat heat pipes (vaporchambers). FTGP may outperform all these research systems as shown inTable 3.

TABLE 3 Comparison between FTGP and some Research Micro Flat Heat Pipes(Vapor Chambers) Hermeticity Wicking Heat Pipe (% fluid (g, Wick ThermalThermal loss/yr inertial Conductivity Conductivity Thickness Area WeightDuration Ref @100° C. force) (W/mK) (W/mK) (mm) (cm × cm) (gm) (hours) IN/A  1 g groove 600 0.7 2 × 2 N/A N/A 2 N/A  1 g silver particle 12,0004 10.8 × 2.65 N/A N/A and mesh 3 N/A  1 g groove 133 2 8 × 3 N/A N/A 4N/A  1 g groove 625 1.25 2.4 × 1.6 N/A N/A 5 N/A  1 g copper mesh 1576.35   12 × 1.27 N/A N/A 6 N/A  1 g Sintered column 1050 1 6 × 2 N/A N/AFTGP 0.1 25 g 100 30,000 1 20 × 40 50 1,000

FTGP may incorporate novel features: a hybrid micro/nano wickingstructure as well as macro/micro/nano fabrication and assemblycompatible with flexible circuit manufacturing technologies. Thefabrication and assembly may include maskless fabrication of the hybridmicro/nano-scaled wicking structures using various nanofabricationtemplates, atomic layer deposition for hydrophilic, hydrophobic,hermetic and corrosion-protective coatings. These techniques andfeatures are to be discussed next, followed by embodiments of differentprocessing and assembly steps.

Embodiments may provide an FTGP with a hybrid micro/nano wickingstructure. Nanotechnology may enable the fabrication of billions ortrillions of nanorods or nanomeshes to increase surface areatremendously and create an opportunity to improve the heat transferperformance of a thermal ground plane substantially. FIG. 8 mayillustrate some physical limitations and some state-of-the-artexperimental data of the critical heat flux that evaporation orcondensation could achieve. The critical heat flux may be the maximumheat flux which the evaporator can sustain before burn out. The physicallimitations may be estimated based on the kinetic theory; they are about1000× higher than the ones reported, even with microstructures. There islarge potential for improvement resulting from nanotechnology.

However, the performance of a thermal ground plane may not be governedby evaporation or condensation processes only. In some embodiments, thesystem's performance may actually be affected by many trade-offconsiderations. FIG. 9 shows a one-dimensional heat transfer model,merely by way of example, which includes the following two performancemeasures:

FTGP Thermal Conductivity=function of (size of nanomesh's opening, sizeof nanorods, size of microchannels, size of micro cubes, contact anglesof hydrophilic and hydrophobic coatings, FTGP length and width, wick'sthermal conductivity and thickness, and vapor core thickness)

FTGP Maximum Allowable Inertial Force for Effective Wicking=function of(size of nanomeshes opening, size of microchannels, size of micro cubes,contact angles of hydrophilic coating)

The condensed liquid may return to the evaporator through themicrochannels. The evaporator, condenser, and microchannels may becovered by nanomeshes that separate the vapor flow from the liquid flow.

FIG. 10 presents some results for some embodiments, merely by way ofexample. FIG. 10 may indicate that nanomeshes with pore diameters of 400nm would result in excellent evaporation for heat transfer coefficientsranging from 2,000 to 10,000 kW/m²K, which is about 10-100× higher thanthe state-of-the-art level. As shown in the figure, the smaller the poresize, the higher the evaporation heat transfer coefficients may be.However, there may be a limitation; boiling becomes unstable when thepore size approaches that of the vapor nucleation sites (˜100 nm). Insome embodiments, a 400 nm size may be used to assure excellent andstable evaporation.

FIG. 11 may present the critical heat flux affected by three limitingfactors of an evaporator: capillary pressure, boiling, and entrainmentfor some embodiments. In a nanostructure with d_(porr)=400 nm, forexample, the critical heat flux may be enhanced significantly, reachingthe level of 1000 W/cm². For some embodiments, this level may be limitedby the capillary pumping pressure resulting from the nanostructure. Forsome embodiments, it may be further enhanced by reducing d_(pore), theboiling limit may drop quickly when the size reaches that of nucleationsites.

For some embodiments, an evaporator's size may be governed by the chipsize. Condenser area may be made to accommodate the FTGP's performanceneeds. The condenser's performance may affect total system performance;it may be adjusted as necessary. In some embodiments, dropwisecondensation in the condenser may be emphasized, which may result in aheat transfer coefficient 30 times larger than film-wise condensation byusing surface-coated hybrid micro/nanostructures.

FIG. 11 shows how critical heat flux of evaporators for some embodimentsmay be affected by three limiting factors: boiling, entrainment andcapillary pressure. All the limits may be increased as the pore sizedecreases. Boiling may be limited by the bubble size associated with thepore size. In some embodiments, it may be stable since 400 nm isconsiderably larger than the size of nucleation sites. Entrainment maylimited in some embodiments by the shear stress between the vapor flowand liquid flow as the operation heat flux increases. Capillary pressuremay be limited in some embodiments by the pumping ability of themicro/nano hybrid wick to circulate the working fluids. Capillarypressure may determine the critical heat flux for some embodiments of anFTGP.

In a heat pipe for some embodiments, the liquid may be supplied by thecapillary force difference between the evaporator and condenser. Inorder to obtain sufficient capillary pressure to assure a steady liquidsupply under high-g acceleration for some embodiments, a nanoscale wickstructure may be used. FIG. 12-a may demonstrate that the wickstructures should be smaller than 8 um to generate enough capillarypressure for the wick structure to remain operational under 25-gacceleration for some embodiments. The chosen 400 nm feature size mayassure FTGP to pass an acceleration test, though other sizes may beutilized for other embodiments.

For some embodiments, the smaller the feature size, the better theperformance may be. FIG. 12 show that microtechnology may be importantfor some embodiments along with nanotechnology. The flow resistance interms of pressure drop may increase significantly when the copper cube'ssize is decreased from 50 to 10 μm. These copper cubes may be used inthe evaporator to form channels supplying liquid to nanomeshes throughtheir pores for some embodiments (see FIGS. 8 and 9, for example). The50 μm cubes may be selected for FTGP embodiments because the associatedpressure drop may be less than the capillary pressure induced bynanomeshes. These figures also illustrate a hybrid micro/nano wickingstructure for some embodiments. A nano structure may be desirable forevaporation and condensation heat transfer for some embodiments.However, a micro structure may minimize the flow resistance for someembodiments.

FIG. 12-a shows for some FTGP embodiments to remain operational at 25 gacceleration, the capillary force may be generated by pores with adiameter less than 8 um. It also shows that the pore size of 400 nmselected for nano-evaporation may generate enough force to overcome the25 g acceleration. FIG. 12-b provides illustration of the magnitudes ofcapillary pressure generated by 400 nm pore and pressure drop (flowresistance) for some embodiments resulting from micro structures. Theflow resistance may be inversely proportional to the size of the coppercubes, Δp˜1/D_(cube) ². For some embodiments, changing from 50 to 10 μmcubes, the resistance may increase by 10×. It also shows that thecapillary pressure generated by 400 nm pores may overcome the flowresistance in micro pores with diameter much larger than 10 um for someembodiments. These figures may show that hybrid micro/nano-scaledwicking structure may be used for some embodiments.

For some embodiments, FIGS. 13-a and 13-b may show FTGP thermalconductivity as a function of its length. Merely by way of example,thermal conductivity may reach 40,000 W/mK with a 40 cm length and 0.6mm vapor core thickness for some embodiments. Merely by way of example,some embodiments may have a thermal conductivity of 30,000 W/mK withh_(e)=2500 kW/m²K. Effective thermal conductivity may be a function oflength. For some embodiments, a thermal conductivity for a specificpurpose may thus be determined by length.

In addition to the length effect, FTGP thermal conductivity may also beaffected by the thickness of vapor core for some embodiments. FIG. 13-bmay show the effect of thickness. The vapor core thickness may play arole in governing the overall performance of FTGP. The vapor pressuremay drop between the evaporator and the condenser for some embodimentsmay become more sensitive to vapor core thickness when the FTGP islonger than 5 cm. For some embodiments, a higher pressure drop mayresult in a higher temperature drop and hence brings much lower thermalconductance during the process of vapor transportation from theevaporator to condenser. Merely by way of example, when the vapor coreis reduced to 0.15 mm, the maximum performance of FTGP may be 5148 W/mK.For some FTGP embodiments, the vapor core thickness may be larger than0.5 mm.

Table 4 below summarizes parameters for some embodiments of a novelmicro/nano wicking structure for FTGP. Merely by way of example,microchannels with 200 μm feature size may serve as liquidtransportation passages between the evaporator and the condenser tominimize the flow resistance.

TABLE 4 Nano Pores in Nanomeshes Copper Micro Cubes MicrochannelsGeometric magnitude 400 nm 50 μm 200 μm

Table 5 below merely provides some comparison in the performance of someembodiments with state-of-the-art configurations. The capillary pressureresulting from evaporation may be enhanced 800× by this wickingstructure for some embodiments. The flow resistance may increase due tothe use of micro cubes; however, for some embodiments, this may bemitigated by only using 50 μm cubes in the evaporator region. In otherregions, 200 microchannels may be used and their pressure drop may besmall. For heat removal capability, a micro/nano structure may enhancethe surface area, increasing the evaporator's effective heat transfercoefficient by 500× for some embodiments.

TABLE 5 Capillary pressure Flow resistance Surface area*${\Delta \; P_{c}} = \frac{4\; {\sigma \; \cdot \cos}\; (\theta)}{d_{pore}}$$\Delta \; {\left. P_{flow} \right.\sim\frac{2\; f\; {Re}_{l\mspace{11mu}}L}{ɛ\; D_{cube}^{2}}}$$\beta = \frac{A_{s}}{V}$ Parameters Contact angle Pore size Largeparticle Length volume ratio Elements (θ) (d_(pore)) (D_(cube)) (L) (β)Existing number [8] cos (60°) = 0.5  100 μm ~100 μm ~40 cm 1.2 × 10³ m⁻¹Proposed number cos (10°) = 0.99 400 nm  ~50 μm  ~5 cm   6 × 10⁵ m⁻¹Component improvement 2 X 400 X 4 X −8x  500 X Net improvement −800 X −2X ~500 X

Embodiments also provide methods for fabrication and assembly of FTGPcompatible with different circuit manufacturing, including but notlimited to flexible circuit manufacturing. Some embodiments mayinvolving the following: a) diblock copolymer for the masklessfabrication of the hybrid micro/nano-scaled wicking structure, b) atomiclayer deposition for hydrophilic, hydrophobic and hermetic coatings, andc) matching of coefficients of thermal expansion (CTE) with a liquidcrystal polymer (LCP) substrate that is selected as the casing materialfor FTGP.

Some embodiments may involve maskless fabrication of hybridmicro/nano-scaled wicking structure. In some embodiments, nanorods andnanomeshes covering the micro-scaled features may be utilized withmicro/nano wicking structure. FIG. 14 may show structural and propertyfeatures for some embodiments. FIG. 14 may include a hybrid micro/nanowicking structure that may be fabricated by a process compatible withflexible circuit manufacturing. Some embodiments may feature wickthermal conductivity. Fabricating hybrid micro/nanostructures directlyout of copper or other metal or thermally conductive material may ensuregood contact between the micro and nanostructures, in addition to itsintrinsic high thermal conductivity (K=400 W/m K) for some embodiments.Merely by way of example, with a reasonable porosity (<0.6), embodimentsmay achieve K_(wick)=100 W/m K. Some embodiments may involving creatinga micro/nano-scaled structure to assure reliable heat pipe operationunder 25 g acceleration. Merely by way of example, a structure with 50μm microcubes and 400 nm nanomeshes may generate pressure sufficient toovercome the pressure load resulting from the 25-g acceleration,anywhere from room temperature up to 80° C. Some embodiments, merely byway of example, may achieve 2,500 kW/m²K for the evaporator's heattransfer coefficient. The liquid may flow through the microcubes tonanomeshes so that the phase change phenomena can happen on top of themicrocubes. For some embodiments, nanorods between microcubes andnanomesh, shown in FIG. 15, may supply a liquid.

However, it may be challenging to fabricate nano-scaled features onmicro-scaled features while being compatible with flexible circuitmanufacturing, in which 25 μm lines/spacings may be typically thesmallest features possible. Diblock co-polymer may be used in someembodiments to address this challenge.

For some embodiments, a block copolymer molecule may contain two or morepolymer chains attached at their ends and can self-assemble to form ananoscale structure with a microdomain. Block copolymer films may beprepared by the spin-coating technique. The film thickness and thesurface roughness may be controlled through the spin speed, theconcentration of the block copolymer solution or the volatility of thesolvent. The volume fraction of the components, the rigidity of thesegments in each block, the strength of the interactions between thesegments, and the molecular weight may contribute to the size, shape,and ordering of the microdomains.

For some embodiments with micro/nano wicking structure, diblockcopolymers composed of polystyrene and polymethylmethacrylate, denotedP(S-b-MMA), may be applied to form a rich variety of nanoscale periodicpatterns and to offer the potential to fabricate high-density arrays.PS-b-PMMA is known to be stable, compatible with currentphotolithography processes and amenable to multilayered devicefabrication. FIG. 15 shows the fabrication process of nanorods usingdiblock copolymers that may be used for electroplating copper nanorodswith etching copper nanomeshes, for some embodiments. Afterspin-coating, Poly(methyl methacrylate) (PMMA) and polystyrene (PS) maybe annealed and electrically aligned. The PMMA molecules may thenremoved by UV exposure and rinse. Merely by way of example, the poresremaining may be in the range between 10 nm and 2 um. Processoptimization may be conducted to reach the target dimension of ˜30 nmfor the nanorods and ˜400 nm for the nanomeshes for some embodiments.The co-polymer template may then be used as a mask to etch nanoscaledfeatures or for electroplating.

Some embodiments may involve Atomic Layer Deposition (ALD) forHydrophilic, Hydrophobic and Hermetic Coatings. For some embodiments,ALD may be important to the hydrophilic and hydrophobic coatings onmicro/nano-scaled wicking structures due to its nanoscale thickness forextremely low intrinsic thermal resistance. Merely by way of example,for the hydrophilic coating's expected ˜0° contact angle in water, theself-assembled monolayer may have hydrophillic functionality using polarchemical groups such as —OH or —(OCH2CH2)nOH [PEG]. ALD Si02 and TiO2coatings may then be applied on nanorods/nanomeshes of the evaporatorfor some embodiments.

ALD alkylaminosilanes polymer may be formed with covalent bonding to thehydroxyl groups on an ALD alumina layer (see FIG. 5, for example)attached to the nanorods/nanomeshes for some embodiments. This ALDprocess developed for MEMS may provide advantages over self-assembledmonolayer. Merely by way of example, the contact angle with water on aplanar surface may be about 108° and can be more than 160° for ALDsuperhydrophobic nanostructures. Moreover, ALD alumina coating mayprovide an excellent water vapor barrier for some embodiments (see FIG.6, for example). The water vapor permeability may be 1.7×10⁻⁵ gm/m² day,which is about 50,000× lower than the 1 gm/m² day for a typical polymer.

For some embodiments, ALD hydrophilic, hydrophobic and hermetic coatingsmay be used. Aging effects on the permeability may be shown in FIG. 6.The ALD alumina may be slowly damaged by the liquid water and highhumidity moisture in some embodiments. However, ALD SiO₂ may survive insuch an environment. Additional ALD SiO₂ or fluoropolymer on top of ALDalumina may be used to for some embodiments.

Producing high quality ALD coatings over copper surfaces may havechallenges. As shown in FIG. 16, many defects may be identified byelectroplating copper onto an ALD-alumina-coated over a copper surface.Copper surfaces may oxidize quickly. For some embodiments, ALD-on-coppercoating may see a reduction in the defect density by at least 1000×.FIG. 16 shows defects that may be identified from ALD-alumina-coatedcopper.

For some embodiments, liquid crystal polymer (LCP) may be used as acasing materials for FTGP. LCP may have low moisture permeability, whichis good for the hermetic sealing needed to assure extremely low fluidloss. Polyimide material commonly used for fabricating flexible circuitboards can be good alternative to LCP.

Referring back to FIG. 7, this shows embodiments with themicro/nano-scaled evaporator structure fabricated on LCP for someembodiments. An evaporator may then be thermally connected to anintermediate dummy chip through thermal vias and copper pads. Theeffective CTE of the dummy chip may be collectively influenced by theLCP, the copper vias, and the chip attachment material. For someapplications, thermal vias and the dummy chip's die attachment may betoo thermally resistive. In such eases, one may apply a configurationwith the evaporator structure directly fabricated on the dummy chip forsome embodiments (see FIG. 7-b) with excellent thermal performance. Thischip may be attached to the LCP flexible circuit for some embodiments.

FIG. 17 shows embodiments of a finite element model in a test case withGaAs attached to a copper pad, which is to be fabricated on the LCPsubstrate. FIG. 17-a shows a 3-D structure of chip-on-chip-oncopper/LCP. FIG. 17-b shows a finite element modeling conducted fordummy chip on copper structure. FIG. 17-c shows a deformation of thedummy-epoxy-copper (structure/stack) (deformation is not to scale. Thetable below may demonstrate that the effective CTE of the GaAs dummychip may be matched through structure design and materials selectionusing epoxy or solder. The table may show the effective CTEs of GaAsdummy chip attached to a copper pad through Au80Sn20 solder and epoxyand CTE matches between the dummy and the real GaAs. chips

GaAs—AuSn-Copper Structure CTE at the top Thermal surface CTE ofExpansion Thickness [μm] of GaAs GaAs Mismatch # GaAs AuSn Copper [ppm/°C.] [ppm/° C.] [%] 1 300 50 300 6.88557 6.9 0.21 2 100 10 100 6.764896.9 1.96 3 100 10 150 6.92355 6.9 0.34 GaAs-Epoxy-Copper Structure CTEat the top Thermal surface CTE of Expansion Thickness [μm] of GaAs GaAsMismatch # GaAs Epoxy Copper [ppm/° C.] [ppm/° C.] [%] 1 500 30 106.740750 6.9 2.31 2 500 25 10 6.796948 6.9 1.49 3 500 20 10 6.882974 6.90.25 4 500 15 10 6.968744 6.9 0.99 5 500 10 10 7.083829 6.9 2.66 6 40010 10 6.787219 6.9 1.64

Some embodiments may involve different fabrication and assembly stepsincluding the following: I) integration of micro structures (thermalvias, microcubes in evaporators and condensers, and microchannels forliquid flow path) and flexible circuit with copper microstructures; II)nanostructure fabrication using diblock copolymer template; III) ALDhydrophilic coating on evaporators and ALD selective hydrophobic coatingon condensers; and IV) assembly process.

Some fabrication and assembly embodiments may involve a stage involvingLCP flexible circuit with copper microstructures. Merely by way ofexample, flexible circuit vendors may provide fast and flexible designof liquid crystal polymer (LCP) substrates with copper through-hole viasand copper microstructures and multilayer flexible circuits. For someembodiments, copper through-hole vias may be processed by laser drillingand subsequent electroplating, as shown in FIG. 18. For multi-functioncircuit applications, multilayer LCP structures may be finished by hotoil press or autoclave.

Some fabrication and assembly embodiments may involve a stage involvingnanowick fabrication. For some embodiments, with a LCP substrate, ablock copolymer template may be used to achieve high density nanorod andnanomesh structures in the condenser and evaporator. To fabricate thedesigned micro/nanostructures as shown in FIG. 14, one may advantage ofthe following attributes of co-polymer templates: 1) the size of thenanostructures can be controlled by the ratio of the copolymers and 2)the co-polymer template can be used for both etching and depositionmasks. The first copolymer template may be used to electroplate coppernanorods and then use the second copolymer template to etch the coppernanomeshes. The sizes of rods may be controlled and optimized. DetailedStage II processes for some embodiments are shown in FIG. 19-a.

FIG. 19-a shows a method of fabrication process of copper nanomeshes oncopper micro structures using co-polymer templates for some embodiments.The process may use the first copolymer template to electroplate coppernanorods and then uses the second copolymer template to etch the coppernanomeshes. The feature sizes may be controlled and optimized.

The method of fabrication process of copper nanomeshes on copper microstructures using co-polymer templates for some embodiments shown in FIG.19-a. The method may include a planarization process by spin-coatingphotoresists and then soft baking at Stage II-a. The method may includespin coating of 1^(st) diblock copolymers (PS-b-PMMA, for example), atStage II-b. At Stage II-c, electrical field alignment and annealing mayoccur. Deep ultraviolet exposure and removal of PMMA may occur at StageII-d. At Stage II-e, electroplating of copper nanorods may occur. Copperfilm deposition by E-Gun evaporation may occur at Stage II-f. Spincoating of 2^(nd) diblock copolymers (PS-b-PMMA, for example) fornano-meshes may occur at Stage II-g. Stage II-h may include electricalfield alignment and annealing. Deep ultraviolet exposure and remove ofPMMA more occur at Stage II-i. Removal of copper film by wet etching andremoval of 2^(nd) copolymer may occur at Stage II-j. Removal of 1^(st)copolymers may occur at Stage II-k. And removal of photoresistor mayoccur at Stage II-l.

FIG. 19-b shows another method of fabrication process involving hybridcopper micro/nano structures. The copper micro/nano-wicking structuremay be fabricated by integration the photolithography and porous anodicalumina (PAA) template techniques, as shown in FIG. 19-b. Photoresist(PR) with designed micro-pattern may be created on the top of copperplate by using the photolithography process, and then the copper can befilled into these micro-pores by electroplating to obtain the orderedcopper micro-pillars. The size of micro-pillars may be relative to thephotolithography mask size, and the length of Cu pillars can becontrolled by adjusting the electro-plating time. After removing thephotoresist in acetone, atomic layer deposition (ALD) coating of a thinAl₂O₃ layer and subsequent absorbing FOB(DMA)S chains may be applied onthe surface of as-prepared copper micro-pillar arrays, which could leadto the formation of hydrophobic surface. Then, photoresist may bespin-coated again to fill the gap between copper micro-pillars, and aflat surface can be obtained after the polishing process. By placing thePAA template on the top of these Cu microchannels and electroplating,free-standing Cu nanowire arrays can be fabricated just on the coppermicro-pillars because of the confinement of PAA nanochannels for someembodiments. In the place of photoresist, no copper nanowire may beformed due to its electrical insulation. The as-obtained copper nanowirearrays may be modified to be hydrophilic by coating a thin layer of TiO₂using ALD process. The copper micro/nanowicking structure with separatedhydrophobic and hydrophilic surface may be obtained.

FIG. 20 illustrate the product after Stage I and Stage 2 for someembodiments. A microcube matrix in the evaporator and condenser (resultsof Stage I) may ensure a continuous liquid supply with low flow frictionresistance both vertically and laterally. Fabricating a continuousnanomesh may sustain in a micro-porous environment, ensuring appropriateoperation under high g acceleration and orders of magnitude enhancementin phase change heat transfer for some embodiments. Obtainingevaporation or condensation at the microcubes may ensure low thermalresistance from case to phase-change site. This may be achieved by anintermediate nanorod layer between the microcubes and the top nanomeshlayer, which may assure flow continuity from the phase-change front atthe evaporator and condenser (results of Stage 1) may ensure acontinuous liquid supply with low flow friction resistance bothvertically and laterally. Fabricating a continuous nanomesh may sustainin a micro-porous environment, may ensure appropriate operation underhigh g acceleration and orders of magnitude enhancement in phase changeheat transfer. Obtaining evaporation or condensation at the microcubesmay ensure low thermal resistance from case to phase-change site. Thismay be achieved by an intermediate nanorod layer between the microcubesand the top nanomesh layer, which assures flow continuity from thephase-change front at the nanostructures to the microwicks for someembodiments. FIG. 20-a shows micro/nano wicking structure for asuper-hydrophilic evaporator. FIG. 20-b shows a micro/nano wickingstructure for selective ALD hydrophobic/hydrophilic coating.

Some fabrication and assembly embodiments may involve a stage involvingALD hydrophilic coating on evaporators and ALD selective coating oncondensers. After hybrid micro/nano wick structures are fabricated,nanoscale-thickness ALD coatings which offer extremely low intrinsicthermal resistance may be coated on the wick structures. ALD alumina maybe coated to cover every feature first for inner hermetical seal. ALDSi0₂ or TiO₂ hydrophilic coatings may then be applied. For thecondenser, ALD hydrophobic coating may be applied with a pattern shownin FIG. 20-b. Such a pattern may promote dropwise condensation on thehydrophobic sites and quickly remove droplet through hydrophilicwicking. Selective ALD coating may be accomplished using photoresists orother sacrificial layer to cover unwanted regions. ALD may be coated ata temperature as low as 50° C., so we will have many options for thesacrificial materials.

Additional stages may involve fabrication of hybrid micro/nano-wickingstructures for evaporators, condensers, and low flow resistance microchannels that can operate under high-g accelerations.

Some fabrication and assembly embodiments may involve a stage involvingan assembly process. The assembly process may include the followingsteps: a) die attach of the dummy chip, b) lamination, c) ALD hermeticsealing, and d) charging.

For some embodiments, after lamination, ALD Al₂0₃ and Si0₂, may beapplied to encapsulate the entire exterior of the assembly. ALD Al₂0₃also may act as an excellent gas diffusion barrier and hermetic seal.The ALD Al₂0₃ may have the ability to nucleate and grow on polymers evenif the polymers do not contain chemical functional groups. The barrierproperties of the ALD Al₂0₃ films on polymers have been excellent. Thewater vapor permeability may be only 1.7×10⁻⁵ gm/m² day, which is about50,000× lower than 1 gm/m² day for a typical polymer for someembodiments. For some embodiments, however, ALD Al₂0₃ may need to beprotected from water damage. The protection may be provided byadditional ALD Si0₂ or fluoropolymer on top of ALD alumina.

Additional Methods for Fabricating a Flexible Thermal Ground Plane

Previous sections of this Application provide means for fabricating aflexible thermal ground plane. Additional means to fabricate a flexiblethermal ground plane are now described.

The flexible thermal ground plane device (FTGP) may serve as a flexibleboard to spread heat generated from high power integrated circuits to alarge area.

Embodiments disclosed below may include the use of micro-meshed coppersheets. As shown in FIG. 21, a piece of commercially available coppermesh may be used as a membrane separating a vapor and a liquid transportregion. In some embodiments, the copper mesh may be replaced bymembranes with nano-scaled features to enhance heat transfer performancein some embodiments.

Methods and means for fabricating a FTGP as disclosed may be implementedas now described. It will be recognized by those of skill in the artthat various modifications, alternative constructions, and equivalentsmay be used without departing from the spirit of the invention.Accordingly, the following description should not be taken as limitingthe scope of the invention.

In one embodiment, a means for fabricating a FTGP may begin with a 100μm (4 mil) thick liquid crystal polymer (LCP) with double sided 18 μmcopper (Cu) lamination. While FIG. 22 reflects a size of a current testvehicle as 6 cm×3 cm, it can be any size the user desires. Otherthickness of LCP and Cu lamination may also be used within the spirit ofthe invention, along with different types of LCP and different types ofconductive lamination may be used.

FIG. 23 shows steps for fabricating a FTGP where one may etch away thecopper to serve as a mask for micro-via formation. The top surface maybe coated with positive photo-resist (PR). The PR may be UV exposed anddeveloped to form a negative mask of the via holes, which are 100-200 μmfor the current test vehicle. Other hole sizes may be created. The piecemay then submerged in liquid copper etchant until the LCP may be seenthrough the top layer of copper. The PR may then removed with acetone orother suitable solvent. FIG. 33 also shows two collections of holes.These holes may represent the eventual placement of an evaporator regionand a condenser region, and may have different overall areas. Merely byway of example, the evaporator region may be 1 cm by 1 cm as shown inFIG. 23. Merely by way of example, condenser region may be 2 cm by 2 cmas shown in FIG. 23. The sizes of evaporator and condenser regions maybe varied depending the different requirements of the FTGP.

FIG. 24 shows additional steps that may be used for fabricating a FTGP.The FTGP vehicle or piece may be submitted to reactive ion etching(“RIE”) to cut micro-vias in the LCP layer. The RIE process may use aCF₄—O₂ plasma etch. Since the etching process may not be uniformthroughout the chamber, each sample may be rotated 180° every hour tomaintain an even height on all vias. Merely by way of example, theprocess may take approximately 10 hours with a 16 ccpm-4 ccpm flow rate.Other process times, flow rates, etching processes and materials, androtations may be used for etching steps.

FIG. 25 shows one cross section view of an embodiment with micro-viasformed in LCP with RIE. Other dimensioned micro-vias may be formed.

FIG. 26 shows additional steps that may be used for fabricating a FTGP.A top layer may be etched or removed and micro-vias may be filled withcopper electroplating. In some embodiments, a tape or mask may beapplied to the bottom and to the via areas first. Then, the piece may beimmersed in liquid copper etchant. Once the top copper may have beendissolved, the tape or mask may be removed from the copper on the viaareas. The piece may then be placed in an etchant again. This mayprevent the etchant from dissolving the bottom layer of copper andcompromising the hermeticity of the final test vehicle.

FIG. 27 shows one embodiment with a cross section of copper filledmicro-vias through LCP.

FIG. 28 shows additional steps that may be used for fabricating a FTGP.The FTGP piece may then be fastened to an acrylic frame and then placedin a copper electroplating apparatus. The frame may prevent distortionof the sample from stresses as the plating proceeds. Merely by way ofexample, it may take approximately 4 hours for the copper to plate abovethe top surface of the LCP layer. At that point, the top of the vias maymeet and form one large pad. The pad may then polished smooth. Merely byway of example, a thin layer (200 nm) of Cu may be evaporated on thepad. It may be electro-plated to make it 18 μm thick and the evaporatorand condenser regions may be patterned with photolithograph and copperetchant. Other thicknesses of Cu may be used in different embodiments.Different electroplating materials may also be used in embodiments.

FIG. 29 shows additional steps that may be used for fabricating a FTGP.To maintain a planar mounting surface for the heat generatingcomponents, thin Cu or other material plates may be fastened toevaporator and condenser sections of the FTGP.

FIG. 30 shows additional steps that may be used for fabricating a FTGP.The bottom layer of copper may be electro-plated to 50 μm thick and maybe patterned with photolithography and copper etchant to form the microchannels (wicking structures). The via pads (now at the bottom) may becovered with tape to prevent damage from the etchant. In someembodiments, different thicknesses for a bottom layer of copper may beused. Different sized micro-channels may also be formed for differentembodiments.

FIG. 31 shows an SEM image of one embodiment with copper micro-cubeevaporator structures formed with photolithography and copper etchant.

FIG. 32 shows additional steps that may be used for fabricating a FTGP.Nano wick structures may be formed on the top surface of the micro flowchannels. Di-block copolymers may be used to fabricated the nanocolumns.Other polymers and materials may be used to fabricate nanocolumns.

FIG. 33 shows additional steps that may be used for fabricating a FTGP.Atomic layer deposition (ALD) coatings may be applied to a micro mesh topromote hydrophobic and hydrophilic behavior of the condensed liquid.

FIG. 34 shows additional steps that may be used for fabricating a FTGP.Copper channels may be coated with evaporated titanium (for adhesion)and then gold. Other materials may be used for these coating steps. Thepiece may then plated with gold and a micro-mesh is bonded to the cubesand channels. The mesh may serve as a wicking structure.

FIG. 35 shows an SEM image of a micro-mesh wicking structure in oneembodiment. Merely by way of example, the holes may be 5 μm with a 11 μmpitch.

FIG. 36 shows a cross section view of bonded Cu mesh to themicro-channels and Cu filled thermal vias for one embodiment.

FIG. 37 shows additional steps that may be used for fabricating a FTGP.To prevent collapse of a flexible polymer cover, acetal polymer spheresmay be strategically placed and bonded to the structure with epoxyresin. Acetal polymer may used because of its low water absorption rate.Other support members may be used in some embodiments. The structure maybe encased with a spacer (FR-4 as an example), topped with an ALD coatedLCP cover. The methods may be used for bonding are either athermosetting (thermo-bonding) polymer or epoxy resin.

FIG. 38 shows the top and bottom view of an a completed FTGP vehicle inaccordance with various embodiments. In some embodiments, the bondingbetween the copper mesh and the cubes may be done by electroplating orby thermo-compression or thermosonic bonding. For hermetic sealing insome embodiments, ALD-based barrier coating may be used to cover thepolymer outside surface. In addition, commercially available polymerfilms with barrier coating may be used as the base material, e.g. liquidcrystal polymer illustrated.

FIG. 38 shows one embodiment of an FTGP that may be used to test thefabrication process and to determine the effective thermal conductivityof an FTGP. A schematic of another embodiment of an FTGP apparatus isshown in FIG. 39 that may also be used for characterization of differentFTGPs, including determining the effective thermal conductivity of anFTGP device.

An FTGP may be evacuated of all non-condensable gasses and then chargedwith a specific amount of de-ionized (DI) water. After charging, thefill tubes may be crimp sealed and then soldered shut. The condenserarea may then be placed in a heat exchanger and a variable power ceramicheater may be fixed onto the evaporator pad. Water may be run throughthe heat exchanger and the heater may be switched on. The temperaturesof the thermocouples (mounted on the FTGP) may be monitored until theyreach a steady state value. At this point, the effective thermalconductivity may be calculated.

Merely by way of example, the performance of a flat polymer heat pipemay be measured by finding the effective thermal conductivity of thedevice. This is done by using Fourier's Law of heat conduction, forexample:

$k_{eff} = {\frac{q_{out}}{A}\frac{\Delta \; x}{\left( {T_{e} - T_{c}} \right)}}$

The device, as shown in FIG. 39 for example, may be mounted in a waterheat exchanger to measure q_(out) which is the heat transferred by theheat pipe to the flow of water in the heat exchanger. The center tocenter distance of the evaporator and condenser may be defined by Δx.The cross sectional area of the heat pipe is A. T_(e) and T_(c) may bethe average temperature of the evaporator and condenser respectively.These temperatures of these sections may be monitored with k-typethermocouples mounted with thermal epoxy in the locations indicated bythe black dots in FIG. 39, for example.

The heat transferred to the water may be calculated by the relation,

q _(out) =mc(T _(out) −T _(in))

The temperatures of the water in, T_(in), and the water out, T_(out),may also monitored with k-type thermocouples. The mass flow rate of thewater may be defined as m and the specific heat of the water may bedefined as c.

Merely by way of example, a 0.7 cm² square ceramic heater may be clampedto the evaporator and supplied the heat to the device.

Merely by way of example, a flat polymer heat pipe testing system asshown in FIG. 39 for example may be used. The electrical power suppliedto the heater may be adjusted to 3, 5, or 7.5 W, for example, and theheat pipe may be oriented either horizontally or vertically with adversegravity (evaporator on top). The device may operated until steady statetemperatures were achieved. FIG. 40 provides some data regarding theeffective thermal conductivity calculated with time.

FIG. 40 may show that the best performance of 850 W/m*K occurred at aninput power of 3 W in the horizontal orientation and the worstperformance, 450 W/m*K, occurred at 5 W in the vertical orientation. Theeffective thermal conductivity may also measured for the heat pipecontaining no charge and heat transfer was due to pure conductionthrough the LCP, FR4, glass, and air, merely by way of example. This maythen compared to the theoretical conductivity of the non-charged heatpipe and this may also be seen in FIG. 40.

Having described several embodiments, it will be recognized by those ofskill in the art that various modifications, alternative constructions,and equivalents may be used without departing from the spirit of theinvention. Accordingly, the above description should not be taken aslimiting the scope of the invention, which is defined in the followingclaims.

That which is claimed:
 1. A method for manufacturing a thermal groundplane, the method comprising: providing a first planar substrate member;disposing a liquid channel on the first planar substrate member; bondinga mesh structure on either or both the first planar substrate member andthe liquid channel; disposing a vapor core on at least one of the firstplanar substrate member, the liquid channel, and the mesh structure,such that the mesh structure separates the liquid channel from the vaporcore; disposing a second planar substrate member on the first planarsubstrate member such that the second planar substrate member and thefirst planar substrate member enclose the liquid channel, the meshstructure, and the vapor core; sealing at least a portion of the firstplanar substrate member with a portion of the second planar substratemember; and charging the thermal ground plane with a working fluid. 2.The method according to claim 1, further comprising bonding two similarfirst planar substrate members with the liquid channel, and the meshstructure together such that the mesh structure separates the liquidchannel from the vapor core;
 3. The method according to claim 1, whereinat least one of the mesh structure, the first planar substrate member,and the second planar substrate member are flexible.
 4. The methodaccording to claim 1, wherein disposing the liquid channel on the firstplanar substrate member further comprises electroplating a plurality ofcopper features on the first planar substrate member.
 5. The methodaccording to claim 4, wherein the plurality of copper features compriseat least one plurality of micropillars, a plurality of microchannels, ora plurality of nanorods.
 6. The method according to claim 1, furthercomprising: disposing a plurality of thermal vias between the liquidchannel and either or both a condenser region and an evaporator region.7. The method according to claim 1, wherein the mesh is bonded byelectroplating with either or both the first planar substrate member andthe liquid channel.
 8. The method according to claim 1, wherein thevapor core includes one or more support structures disposed between themesh and the second planar substrate.
 9. The method according to claim1, wherein the vapor core includes one or more spheres disposed betweenthe mesh and the second planar substrate.
 10. The method according toclaim 1, further comprising applying a coating to at least one of themesh structure, the first planar substrate member, and the second planarsubstrate member, wherein the coating creates at least one hydrophobicregion.
 11. The method according to claim 1, further comprising applyinga coating to at least one of the mesh structure, the first planarsubstrate member, and the second planar substrate member, wherein thecoating creates at least one hydrophilic region.
 12. The methodaccording to claim 1, further comprising applying a coating to the meshstructure, wherein the coating creates corrosion protection for thethermal ground plane.
 13. A thermal ground plane comprising: a firstplanar substrate member; a plurality of features electroplated to thefirst planar substrate member; a mesh structure bonded on either or boththe first planar substrate member and the plurality of features; a vaporcore disposed on at least one of the first planar substrate member, theplurality of features, and the mesh structure, wherein that the meshstructure separates the liquid channel from the vapor core; and a secondplanar substrate member disposed on the first planar substrate membersuch that the second planar substrate member and the first planarsubstrate member enclose the plurality of features, the mesh structure,and the vapor core; wherein at least a portion of the first planarsubstrate member is sealed with a portion of the second planar substratemember, and wherein the thermal ground plane is charged with a workingfluid.
 14. The thermal ground plane according to claim 13, wherein theplurality of features comprise a plurality of micropillars, a pluralityof microchannels, or a plurality of nanorods
 15. The thermal groundplane according to claim 13, wherein the mesh comprises a nanomeshstructure etched on the plurality of features.
 16. The thermal groundplane according to claim 13, wherein the mesh is bonded byelectroplating with either or both the first planar substrate member andthe plurality of features.
 17. The thermal ground plane according toclaim 13, wherein the vapor core includes one or more support structuresdisposed between the mesh and the second planar substrate.
 18. Thethermal ground plane according to claim 13, further comprising aplurality of thermal vias disposed between the liquid channel and eitheror both a condenser region and an evaporator region.
 19. A thermalground plane comprising: a first planar substrate member; a plurality ofnanorods electroplated to the first planar substrate member; a nanomeshstructure disposed on at least a subset of the plurality of nanorods; avapor core disposed on at least one of the first planar substratemember, the plurality of nanorods, and the nanomesh structure, whereinthat the nanomesh structure separates the plurality of nanorods from thevapor core; and a second planar substrate member disposed on the firstplanar substrate member such that the second planar substrate member andthe first planar substrate member enclose the plurality of nanorods, thenanomesh structure, and the vapor core; wherein at least a portion ofthe first planar substrate member is sealed with a portion of the secondplanar substrate member, and wherein the thermal ground plane is chargedwith a working fluid.
 20. The thermal ground plane according to claim19, further comprising a plurality of thermal vias disposed withineither or both the second planar substrate member and the first planarsubstrate member.